This invention relates generally to aircraft gas turbine engines, and more specifically to counter-rotating gas turbine engines.
At least one known gas turbine engine includes, in serial flow arrangement, a forward fan assembly, an aft fan assembly, a high-pressure compressor for compressing air flowing through the engine, a combustor for mixing fuel with the compressed air such that the mixture may be ignited, and a high-pressure turbine. The high-pressure compressor, combustor and high-pressure turbine are sometimes collectively referred to as the core engine. In operation, the core engine generates combustion gases which are discharged downstream to a counter-rotating low-pressure turbine that extracts energy therefrom for powering the forward and aft fan assemblies. Within at least some known gas turbine engines, at least one turbine rotates in an opposite direction than the other rotating components within the engine
At least one known counter-rotating low-pressure turbine has an inlet radius that is larger than a radius of the high-pressure turbine discharge. The increased size of the inlet radius accommodates additional stages within the low-pressure turbine. Specifically, at least one known counter-rotating low-pressure turbine includes an outer turbine having a first quantity of low-pressure stages that are rotatably coupled to the forward fan assembly, and an inner turbine having an equal number of stages that is rotatably coupled to the aft fan assembly.
During engine assembly, such known gas turbine engines are assembled such that the outer turbine is cantilevered from the turbine rear frame. More specifically, the first quantity of stages of the outer turbine are each coupled together and to the rotating casing, and the outer turbine is then coupled to the turbine rear frame using only the last stage of the outer turbine, such that only the last stage of the outer turbine supports the combined weight of the outer turbine rotating casing. Accordingly, to provide the necessary structural strength to such engines, the last stage of the outer turbine is generally much larger and heavier than the other stages of the outer turbine. As such, during operation, the performance penalties associated with the increased weight and size may actually negate the benefits of utilizing a counter-rotating low-pressure turbine.